This invention relates to antennas and, more particularly, to a system for transmitting and receiving circularly or elliptically polarized waves and a method for receiving such waves over two widely spaced frequency bands.
At the present time, the FCC (Federal Communications Commission) is considering broadcasting television signals in circular polarization. It has been suggested that improved television reception in large metropolitan areas can be achieved even though the reception antennas are arranged to receive horizontal polarization.
The real advantages, however, of a circularly polarized antenna system can be realized with circularly polarized receiving antennas. It is a well established phenomenon that reflected circularly polarized waves from objects located in the main reception direction such as a building appears at the receiving antenna reversed in rotation. Therefore, it would be possible to reject reflected waves of reversed polarization rotation from the main wave by having a receiving antenna in which the polarization is designed to match the polarization rotation direction of the transmitted wave. For example, a receiving antenna designed to match a right circularly polarized broadcasting wave will be isolated from the reflected waves from the same direction which would appear left circularly polarized.
In the recent tests for circular polarization for broadcasting television, the type of circularly polarized receiving antennas used have been crossed dipoles fed in phase quadrature. This quadrature phasing is provided, for example, by a delay line wherein the feed line length to one of the dipoles in one-quarter wavelength longer than the feed line length to the other dipole where the one-quarter wavelength is measured at the center of the broadcast frequency band. Although this system operates well for a given frequency band, the receiving antenna is frequency limited and as such would not operate to provide reception of, for example, right circularly polarized waves over both the low and high VHF television frequency bands. The system planned, therefore, would require a separate antenna for each of the bands if one were to receive both high and low VHF television frequency bands.